An organic luminescent device is a device where a thin film containing a fluorescent organic compound or a phosphorescent organic compound is sandwiched between an anode and a cathode; an electron and a hole injected from the respective electrodes generate an exciton of the fluorescent compound or the phosphorescent compound; and light emitted when the exciton returns to a ground state is utilized.
According to a study of Kodak company in 1987 (Appl. Phys. Lett. 51, 913 (1987)), there has been reported a luminescence of approximately 1000 cd/m2 at an applied voltage of approximately 10 V in a device having a separated-function type two-layer structure using ITO as an anode, a magnesium-silver alloy as a cathode, an aluminum quinolinol complex as an electron transport material and a luminescent material, and a triphenyl amine derivative as a hole transport material. The related patents include U.S. Pat. Nos. 4,539,507, 4,720,432, 4,885,211, and so on.
In addition, it is possible to generate a luminescence from ultraviolet to infrared ones by changing the kinds of the fluorescent organic compound, and in recent years, extensive studies have been made on various kinds of compounds. For instance, such compounds are disclosed in U.S. Pat. Nos. 5,151,629, 5,409,783 and 5,382,477, Japanese Patent Application Laid-Open Nos. 2-247278, 3-255190, 5-202356, 9-202878, 9-227576, and so on.
In recent years, many studies have been made on an application of energy in a triplet state to an EL using phosphorescent compounds as luminescent materials. A high luminous efficiency exhibited by an organic luminescent device using an iridium complex as a luminescent material has been reported by a group at Princeton University (Nature 395, 151 (1998)).
Furthermore, in addition to the organic luminescent device using a low molecular weight material as mentioned above, an organic luminescent device using a conjugated polymer has been reported by a group at Cambridge University (Nature, 347, 539 (1990)). In this report, a luminescence from a single layer is confirmed through film formation of polyphenylene vinylene (PPV) using a coating system.
The related patents of the organic luminescent device using the conjugated polymer include U.S. Pat. Nos. 5,247,190, 5,514,878, 5,672,678, Japanese Patent Application Laid-Open Nos. 4-145192 and 5-247460, and so on.
In this way, a recent progress in the organic luminescent device is remarkable, and characteristics thereof suggest a possibility of applications for various purposes, which enable the luminescent device with a high luminance even at a low applied voltage, a wide variety of luminous wavelengths, a high-speed response, and a thin and lightweight form.
However, an optical output with a higher luminance or higher conversion efficiency is required under the present conditions. In addition, many problems still remain to be solved regarding a durability against a change with time due to a long-term use, deterioration caused by an atmospheric gas containing oxygen, moisture, or the like. Furthermore, it is not still insufficient for solving problems related to the needs for luminescences of blue, green, and red having high color purity when considering the applications to a full color display and the like.